Lithium secondary batteries are anticipated to become high-output power sources suitable for installation in vehicles. As the negative electrode active material of such secondary batteries, typically graphite carbon, amorphous carbon or other carbon-base materials, lithium-transition metal oxides (Li4Ti5O12 and other lithium titanium oxides and similar), lithium-transition metal nitrides, and similar are used. Such active materials store and release lithium ions through intercalation reactions (that is, reactions to cause inclusion of different molecules and ions between inorganic crystalline layers). However, using such active materials, lithium storage capacity is limited by the crystal structure of the material, so that dramatic increases in capacity cannot be expected. For example, when graphite carbon is used as the negative electrode active material, one Li atom is intercalated per six carbon atoms, so that the charge/discharge capacity has an upper limit of 372 mAh/g. Hence various negative electrode materials which may realize charge/discharge capacities equal to or greater than those of current negative electrode materials (for example, 1000 mAh/g or higher) have been studied.
In recent years, it has been reported that Fe2O3, CoO and other transition metal oxides, as well as NiP2 and other phosphides, can function as active materials (for example, negative electrode active materials) with extremely high capacities (see for example Patent Reference 1). For example, in an electrode comprising Fe2O3 as an active material (an iron oxide-base electrode), a charge/discharge capacity of 1000 mAh/g or higher can be exhibited. Transition metal compounds such as the abovementioned have high capacities compared with conventional oxide-base materials (for example, Li4Ti5O12 and other lithium-transition metal compounds) using intercalation reactions, and can attain higher capacities through the use of reduction reactions from the compound state to a simple metal.
When the transition metal compounds are electrochemically reduced, the oxygen atoms (or phosphorus atoms) which had been bonded with the transition metal react with Li+ ions to form Li2O (or Li3P), and the transition metal element itself becomes a simple metal. Further, upon electrochemical oxidation the Li2O (or Li3P) is decomposed, returning to Li+ ions. Through such electrochemical reactions, the transition metal compounds make possible reversible electrode reactions. As an example of such electrode reactions, an electrode reaction of an oxide MxOy of a transition metal element M such as Fe, Co or similar is indicated in reaction equation (1) below. Reactions such as the abovementioned in which a transition metal compound (typically an oxide) and a lithium ion are replaced with the simple metal and lithium compounds are called conversion (type) reactions.MxOy+2yLi++2ye−←→xM+yLi2O  (1)